Recovery of Lead from Battery Sludge by Electrowinning E. R. Cole, A. Y. Lee, and D. L. Paulson
SUMMARY Research by the Bureau of Mines has resulted in a combination electrorefining-electrowinning method for recycling the lead from scrap batteries. The lead metal grids and lugs are separated from the sludge by ball milling, washing, and screenin-g and are melted and cast into anodes for electrorefining by the Betts process using waste fluosilicic acid as the electrolyte.1 The Betts process, established in commercial use for 80 years, has been well described in the literature2,3 and needs no further description here. The sulfate-oxide-metal sludge remaining after separation of the lead metal is treated in a two-step leaching operation with ammonium carbonate, waste fluosilicic acid, and Pb powder to solubilize the lead for recovery by electrowinning. Unlike electrorefining, electro winning is not being practiced commercially now. Prior attempts to electrowin lead have failed because large quantities of insoluble lead dioxide are deposited on the anodes at the expense of lead deposition on the cathodes. This paper describes bench-scale (1 to 2 liter cells) research for recovering lead from battery sludge by electrowinning, that prevents Pb02 formation at the anodes. The bench-scale work has been successfully completed, and electrowinning experiments are presently being conducted in a 20-liter multielectrode cell.
Table I: Average Composition of Battery Sludge Material, wt.%
One objective of the Bureau of Mines' Minerals and Materials Research program is new technology to promote the secondary recovery of metals, minerals, and other values from waste products. In line with this objective, research was initiated to develop an economic and environmentally acceptable alternative to the pyrometallurgical processes currently used by the secondary lead industry. The secondary industry is the largest producer of lead in the Un.ited States, accounting for - 58% of domestic production. More than 90% of the metal supplied to secondary smelters is in the form of scrap lead-acid batteries. The lead metal and the sludge are separated from the case and the electrolyte and are smelted at high temperatures in a reverberatory or blast furnace. 4 Emissions of lead and sqlfur oxide fumes durin~ pyrometallurgical smelting are very hard to control, and it will be difficult and costly to meet the emission and ambient air standards recently promulgated by the EPA5,7 and OSHA.6 The smelters have an additional burden, which is 'to produce a product that is purer than ever before for use in the new maintenance-free batteries. 8 The electrolytic method devised in this investigation for recycling the lead from scrap batteries eliminates the toxic emissions associated with the present pyrometallurgical smelting process and produces lead pure enough for use in maintenance-free batteries.
MATERIALS, EQUIPMENT, AND PROCEDURES Sludge The battery sludge used in this investigation was obtained from a large domestic secondary smelter and was typical of that charged to their reverberatory furnaces. The average composition of the oven-dried sludge is given in Table I.
Electrodes Graphite anodes were used initially, but later they were replaced with insoluble lead-dioxide-coated titanium (Pb02-Ti) anodes developed at the Rolla Research Center.9 The anodes were 0.5 cm thick by 4 cm wide by 12 cm high. The graphite anodes were solid, but the Pb02-Ti anodes had evenly spaced, approximately 1-cm-diameter holes throughout the surface to aid adherence of the Pb02 coating and electrolyte circulation. Cathodes, which measured 5 cm wide by 15.5 cm long, were cut from O.l-cm-thick cpIToding-grade lead sheet. 42
JOURNAL OF METALS· August 1983
Electrolyte Electrolyte for electrowinning was prepared by a two-step leaching method that converted the battery sludge to PbC03 and PbO, both soluble in H2SiF6, by the process outlined in Figure 1. In a typical leach, in the first step, 100 g (dry weight) of lead sludge was reacted with 30 g of (NH4hC03 and 600 ml H20 at 55-60 a C for 1 h. The (NH4hC03 reacted with the insoluble PbS04 to form acid-soluble PbC03 and (NH4)2S04. After solidliquid separation in step 2, the residue (containing Pb, PbC03, and unreacted Pb02· from step 1) was leached with 500 ml of waste H2SiF6 (specific gravity 1.232) and 14.5 g of -200 mesh lead powder at - 50a C for 1 h. The lead powder converted the insoluble Pb02 to acid-soluble PbO. After solid-liquid separation and diluting to 1 liter, the resulting solution had the desired concentration of - 70 gil Pb and - 90 gil H2SiF6 (free acid) for electrowinning. A typical analysis of the waste acid is given in Table II. The waste acid is formed when fluorine-bearing phosphate rock is treated by either a thermal or an acid process, the fluorine being evolved as SiF4. As the SiF4 is collected in wet scrubbers, it hydrolyzes to give Si04 and H2SiF6. This H2SiF6 is presently being neutralized and disposed of in a landfill. Estimates are that several hundred thousand tons are discarded annually. 10 Initially, aloes and calcium lignin sulfonate were added to the electrolyte as leveling agent and grain refiner, respectively. Aloes is a natural plant extract imported from South Africa, and lignin is a by-product of pulp and paper manufacture. Later, bone gelatin was substituted for aloes as the leveling agent. Electrolysis
Pb(99 99+ pet)
Figure 1. Battery sludge leaching and electrowinning flow diagram.
Table II: Typical Analysis of Waste H2SiF6*
All electrolysis was performed galvanostatically in 1- and 2-liter plastic cells using a 40-V, 50-A dc power supply. A chart recorder and an amperehour meter were used to monitor important operating parameters during the experiments. A Luggin capillary assembly containing PbSiF6-H2SiF6 electrolyte and a reference electrode made from a rod of refined lead, as described by Kerby and Jackson,11 was used to monitor cathode polarization. RESULTS AND DISCUSSION Battery Sludge In addition to the lead grids and lugs, the balance of the lead in batteries is in the form of a lead and lead dioxide paste, or active material, that is applied to the grids. This active material, which has a very fine particle size to promote quick reaction rates, is the part of the battery that reacts with the sulfuric acid electrolyte to store and release energy according to the following equation: Pb02 + 2H2S04 + Pb
2PbS04 + 2H20 + electrical energy
Component H 2SiFs (fluosilicic acid) N0 3 (nitrate) P205 (phosphorus pentoxide) Phosphorus S04 (sulfate) Sodium Iron Aluminum Calcium Magnesium
* The acid also contained trace amounts «0.001%) ofB, Cr, Cu, K, Mn, Mo, Ni, Pb, Ti, and V.
Any unreacted active materials, plus the discharge-charge reaction products, plus corroded or reacted grid metal, are what is referred to here as sludge. The composition and amount of this sludge largely depend on the number of discharge-charge cycles and the length of time the battery has been out of service. Using the specific gravity (1.275) of electrolyte in a fresh battery and the specific gravity (1.125) of electrolyte in a fully discharged battery and noting that the average battery contains about 4 liters of electrolyte, the amount of sot reacted in full discharge is calculated to be 1.08 kg. If the average battery paste contains approximately 2.4 kg each of Pb and Pb02,12 and assuming equal reaction of SO~- with Pb and Pb02, the average battery sludge would weigh about 5.71 kg and would contain approximately 1.06 kg Pb02, 1.24 kg Pb, and 3.71 kg PbS04 If the battery sits for any appreciable time after discharge, further reaction of the lead components would occur and the percentage of sulfate would increase while the percentage of Pb and Pb02 decreased. Sludge Leaching After extensive experiments, the two-step leaching procedure outlined in Figure 1 was developed to solubilize the lead in the sludge for recovery by electrowinning. Equations 2 (step 1) and 3 (step 2) indicate the reactions occurring during the leaching operation: Sludge Residue 1 Solution 1 (Pb, Pb02, PbS04) + (NH4hC03 + H20 ~ (Pb, Pb02, PbC03) + (NH4hS04; (2) JOURNAL OF METALS· August 1983
Residue 1 . Solution 2 . (Pb, Pb02, PbC03) + H2SIFs + Pb powder~ (PbSiFs, H2SiFs) + ResIdue 2. (3) In step 1, the conversion of PbS04 to PbC03 was nearly quantitative when a 100% excess of the stoichiometric amount of (NH4hC03 was reacted with the sludge at 55-60°C for 1 h. Further work is being performed to determine the optimum conditions for crystallization of the (NH4hS04 from solution 1 and its suitability for use as fertilizer. In addition to (NH4hS04, solution 1 contained approximately 10 ppm each of arsenic and lead. Included in this further work are efforts to recycle unreacted (NH4)2C03 and decrease the concentrations of arsenic and lead. Overall recovery of lead by sludge leaching typically ran 92-93%. Repeated attempts to improve these results were unsuccessful. Examination of residue 2 revealed that it was mainly PbS04. This at first seemed to indicate that the conversion of PbS04 to PbC03 in step 1 was incomplete. However, the problem was traced to the waste acid, which was found to contain sulfate ions (Table II). During leaching of residue 1 (in step 2) the sulfate ions in the waste acid converted some of the lead in the solution to insoluble PbS04. Residue 2 is now being recycled to step 1 to convert this additional PbS04 to PbC03.
Operating Parameters Table III: Conditions for Electrowlnning Pb Leached from Scrap Battery Sludge
Parameter Value Selected
Electrolyte composition, gil Lead H2SiF6 Additive concentration, gil Bone gelatin Calcium lignin sulfonate Current density, Alm2 Electrolyte temperature, °C Electrode spacing, cm Cycle time, days
70 90 0.02-0.04 4.0 170-600 28-35
Using electrolyte prepared by leaching battery sludge as described, two Pb02-Ti anodes, and a lead cathode, a series of experiments were initiated to determine the best operating parameters and these are given in Table III.
Electrolyte Composition. The starting electrolyte concentration was 70 gil Pb as PbSiFs and 90 gil free H2SiFs, the same as used for electrorefining. During electrowinning with inert anodes, the concentration of lead in the electrolyte continually decreased and the concentration of acid increased. Additive Concentration. Based on data in a landmark paper by Krauss,I3 cathode polarization measurements were monitored with a Luggin capillary and controlled between 60 and 100 mV to give a smooth cathode deposit. Krauss' work has shown that polarization values greater than 100 m V result in a deposit with "wire growth," and at values less than 60 mY, the deposit has a rough nodular structure. Accordingly, it was determined that 0.6 gil aloes and 4.0 gil calcium lignin sulfonate were the optimum concentrations of addition agents to control cathode polarization within the desired range and give a level cathode deposit. Later, a lowmolecular-weight bone gelatin was used to replace the aloes, a rather costly imported product. This gelatin was a white free-falling powder with an amino acid profile basically the same as that of other bone gelatins. The best combination of addition agents for electrowinning was found to be 0.02-0.04 gil bone gelatin and 4.0 gil calcium lignin sulfonate. Current Density. The initial current density (CD) used for electrowinning was that used for electrorefining, 170 A/m2 . In recent tests to determine the limiting CD, CD's as high as 600 A/m 2 had no adverse effects on cathode morphology. Current efficiency actually increased - 2%, from 96% to 98% at the higher CD. The CD actually used would depend more on the size of the rectifier and tpe number and size of cathodes needed to give the desired production rate than on a limiting CD. Electrolyte Temperature. A minimum electrolyte temperature of 28°C was found to be desirable to minimize power consumption and to produce a smooth cathode deposit. During electrolysis in warm weather at high CD's, the minimum temperature was easily maintained; during cold weather and using low CD's «200 A/m2), it was necessary to add some heat to the cell. Electrode Spacing. Electrodes are -usually spaced as close as possible to minimize energy consumption; however, too close a spacing results in excessive shorting. Industrial experience in lead electrorefining has shown that 1.9 cm clearance between cathode and anode is required at the end of the cycle to minimize shorts. A spacing of 3 cm between anodes and cathodes, the same as for electrorefining, was found to give the proper balance between energy consumption and number of electrode shorts for the cycle times used for electrowinning. Cycle Time. Cycle time varied with current density from approximately 1.7 days at 600 A/m 2 to 6 days at 170 Alm2 . The factor for determining cycle time in electrowinning was the time required to reduce the lead concentration to approximately 25 gil. As the lead content was reduced to <25 gil, granular and finally spongy lead was deposited, but the cathode still analyzed 99.99+% Pb.
JOURNAL OF METALS· August 1983
Anodes The graphite anodes used during the initial tests deteriorated rapidly during electrowinning, and they were replaced with patented Pb02-Ti anodes developed at the Rolla Research Center.9 The Pb02-Ti anodes were found to be particularly suited to electrowinning; unlike the graphite anodes, they did not deteriorate during electrolysis. A pair of these Pb02-Ti anodes were used for more than 42 days accumulative electrowinning time, and they still appeared to be like new. The potential for recovering lead by electrowinning has always been great, but the method has never been used commercially because no one has been able to solve the problem of excessive Pb02 formation at the anodes. The problem was solved during this investigation by the discovery that small amounts of phosphorus in the electrolyte will prevent Pb02 formation. Using electrolyte prepared with RG acid containing no phosphorus, the amount of insoluble Pb02 deposited on the anodes equaled the amount of lead deposited at the cathode. Using the same solution with 1.5 gil phosphorus added as H3P04, approximately 1 g of Pb02 was deposited at the anodes and 100 g of lead was deposited at the cath(j)de during a one-day test. Figure 2 illustrates the remarkable effect of phosphorus in the electrolyte on Pb02 formation at the anodes. In addition to H3P04, a number of other inorganic acids, as well as organic acids, salts, and oxides containing phosphorus, were tested; and all were found to inhibit Pb02 formation at the anodes. However, the fact that the waste acid already contains phosphorus (Table II) makes it particularly suited to preparing electrolyte for electrowinning.
8~ "'''' '"'"0 t3~
z Z 40 00 00
'"'" "'>Ui 20 :~ It:
Figure 2. Effect of phosphorus concentration on Pb02 formation.
Cathode Quality Excellent cathode deposits were produced in all electrowinning tests, first with 0.6 gil aloes and 4.0 gil calcium lignin sulfonate as addition agents, and later with 0.02-0.04 gil bone gelatin substituted for the aloes. Current efficiencies averaged 96%. The lead produced in three-day tests was purer than that produced during electrorefining and averaged 99.99+%. As electrolyte was recycled after electrowinning to leach more sludge, the Luggin capillary was used to monitor cathode polarization and bring the concentration of addition agents up to the proper level to control the polarization voltage within the desired range of 60-100 mV. The data in Table IV indicate how the electrolyte changed in composition during electrowinning and recycling. During electrowinning, the lead concentration decreased and the acid concentration increased from the nominal values as lead was removed from the electrolyte at the cathode. Also, the concentration of Cu, Sb, and Sn increased during recycle of electrolyte. However, copper was the main impurity to codeposit with the lead (Table V). In addition to copper, the cathode from test 6 also showed an increase in phosphorus concentration. The amount of phosphorus in the electrolyte should be easily controlled, as phosphorus is removed in the as-yet-
Table IV: Change in Composition of Recycled Electrolyte
Number of Times Electrolyte Was Recycled
Composition of Electrolyte, gil Pb
Free H 2 SiF6
4B 4A 5B 5A 6B 6A
0 0 1 1 3 3
68.37 10.15 81.22 33.15 84.8 35.8
0.07 0.07 0.07 0.06 0.06 0.06
0.005 <0.001 0.008 <0.001 0.007 0.002
2.59 2.54 2.65 2.67 2.77 2.74
0.38 0.37 0.59 0.57 0.79 0.81
0.03 0.03 0.06 0.04 0.07 0.09
86.6 135.5 88.32 116.64 88.6 138.7
* Suffix B indicates before electrowinning; Suffix A indicates after electrowinning.
Table V: Chemical Analysis of Electrowon Lead
Test 4 5 6
Number of Times Electrolyte Was Recycled
o 1 3
JOURNAL OF METALS • August 1983
Composition, wt. % Pb
99.99+ 99.99+ 99.99+
<0.001 <0.001 <0.001
0.003 0.007 0.007
0.001 0.001 0.004
<0.001 <0.001 <0.001
<0.001 <0.001 <0.001
unexplained reaction at the anode that inhibits Pb02 formation. Auger electron spectroscopy analysis indicates that phosphorus is present on the Pb02-Ti anode surface after electrowinning.
ABOUT THE AUTHORS
E. R. Cole, research supervisor, U.S. Bureau of Mines, Rolla Research Center, Rolla, Missouri
Energy Consumption The thermodynamic energy requirement for electrowinning, unlike that for electrorefining, is a large part of the total energy requirement. The cathodic and anodic reactions during electrowinning are given in equations 4 and 5, respectively, and the overall reaction is given in equation 6:
Dr. Cole received his BS (1966) and PhD (1970) in metallurgical engineering from theUniversity of Missouri-Rolla. His research interests are in the extractive metallurgy of lead and zinc with emphasis on electrometallurgy. Dan L. Paulson, research director, U.S. Bureau of Mines, Rolla Research Center, Rolla, Missouri 65401. Mr. Paulson received his BS in applied science from Portland State University, Portland, Oregon. Since starting with the Bureau of Mines in 1962,. his work has included phase diagram development, refractory carbide evaluations, electric furnace smelting, and hydrometallurgy processing. He is a member of The Metallurgical Society of AIME.
2PbSiFs + 4H+ + 4e- . 2Pb + 2H2SiFs
02 + 4H+ + 4e- . 2H20
2PbSiFs + 2H20 . 2Pb + 2H2SiFs + 02
= -1.36 V
If the reasonable assumption is made that both PbSiFs and H2SiFs are highly ionized, since PbSiFs is soluble in the solution and H2SiFs is a strong acid, the standard potential for the cathode reaction is essentially the reduction potential for lead; i.e., Pb++ + 2e-' Pb, eO = -0.126 V. The standard overall cell potential (e\), and thus the thermodynamic energy contribution, is a large part of the total energy requirement for electrowinning. Given the same electrolyte, current density, electrode spacing, and electrolyte temperature, the total energy requirement for electrowinning for a three-day cycle averaged approximately 8-10 times more than for electrorefining, or - 0.7 kWh/kg versus 0.07-0.09 kWh/kg refined lead. Unlike electrorefining, in which a growing slime blanket caused a constantly increasing potential, the overall cell potential for electrowinning was relatively constant with time at - 2.5 V.
Agnes V. Lee, chemical engineer, U.S. Bureau of Mines, Rolla Research Center, Rolla, Missouri 65401.
Mrs. Lee received her BS in chemical engineering from the National Taiwan University and her MS in chemical engineering from the University of British Columbia. Her research work has been in the fields of electrometallurgy, hydrometallurgy, and soldering since she joined the Bureau of Mines in 1970.
The bench-scale work described in this report for recovering the lead from scrap battery sludge by electrowinning shows a great deal of promise. Lead of sufficient purity for the manufacture of maintenance-free batteries was produced with minimal pollution and relatively low energy consumption. A particular advantage was the ability to use waste H2SiFs as the electrolyte. Because of the discovery that small amounts of phosphorus in the electrolyte will prevent Pb02 formation at anodes, lead electrowinning could become a viable commercial process.
References 1. E. R. Cole, A. Y. Lee, and D. L. Paulson, "Electrolytic Method for Recovery of Lead From Scrap Batteries," U .S. Department of Interior, Bureau of Mines, Report of Investigations 8602, (1981). 2. C. L. Mantell, Industrial Electrochemistry, McGraw Hill, New York, (1950), pp. 314-322. 3. W. H. Dennis, Metallurgy of the Non-Ferrous Metals, Pitman and Sons Ltd., London, England, (1961), pp. 273· 279. 4. K. D Libsch and M. E. Erneta, "Secondary Lead Processing-Current Status," presented at SME·AIME Fall Meeting, St. Louis, Missouri, October 19·21, 1977. Published as Lead-Zinc Update, edited by D. O. Rausch, F. M. Stephens, Jr., and B. C. Mariacher, appendix, p. 13 5. "EPA Issues Final Air Quality Standards for Pb," Chemical and Engineering News, 6, October 9, 1978, p. 6. 6. U.S. Code of Federal Regulations. Title 29-Labor; Chapter XVII-Occupational Safety and Health Administration, Department of Labor; Part 1910--0ccupational Safety and Health Standards, Occupational Exposure to Lead. Federal Register, Volume 43, No. 220, November 14, 1978, pp. 52952-53014. 7. "Proposed EPA Air-Lead Standard Would Ruin U.S. Lead Industry, Report Shows," Mining Engineering, 30, (5) (1978), p. 453. 8. J. H. Thrash, "Maintenance Free Batteries-Effects on the Recycling Industry," Recyclinl( Today, 16, (5) (1978), pp. 36-40. 9. L. L. Smith, R. G. Sandberg, and E. R. Cole, U.S. Department of Interior, U.S. Patent No. 4,159,231, June 16, 1979. 10. H. E. Blake, Jr., W. S. Thomas, K. W. Moser, J. L. Reuss, and H. Dolezal, "Utilization of Waste Fluosilicic Acid," U.S. Department of Interior, Bureau of Mines, Report oflnvestigations 7502, (1971). 11. R. C. Kerby and H. E. Jackson, "Organic Levelling Agents for Electrolytic Lead Refining," Trans. CIM, Annual Volume, 1978, pp. 125·131. 12. A. E. LaPoint, "Recovery of Lead and Lead Compounds From Scrapped Lead-Acid Batteries," Master's Thesis, New Mexico Institute Mining and Technology, Socorro, New Mexico, 1972, p. 122. 13. C. J. Krauss, "Cathode Deposit Control in Lead Electrorefining," J. of Metals, 28, (11) (1976), pp. 4-9.
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